In other words, the "equilibrium of various software products none of which are strictly superior or inferior to each other" is an evidence against the claim that a language X is 10 times more productive than other languages. Or if it is more productive in some areas, then it must have a huge disadvantage somewhere else.

Yup! It's the old 'if you're so good, why aren't you rich' question in more abstract guise. Of course, in the real world, new languages are being developed all the time, so a workable answer is already 'I'm not rich because I'm so new, but I'm getting richer'. This is the sort of answer an up and coming language like Haskell or Scala or Go can make.